Enhanced near-surface nutrient availability and new production resulting from the frictional decay of a Gulf Stream warm-core ring

Abstract

Warm-core eddies in at least three different current systems have been observed to evolve over a few weeks' time from local minima to local maxima in phytoplankton biomass and primary productivity. The causes of this biological enrichment have been uncertain. We report here a possible explanation of this phenomenon, based on events observed in Gulf Stream warmcore ring 82-B. During the time when a phytoplankton biomass maximum was present in this ring, nutrient concentration gradients beneath a nutrient-depleted surface layer were consistently both greater in magnitude and closer to the sea surface in the interior of the ring than in the surrounding Slope Water. In addition, surface-layer densities were greater inside than outside the ring, resulting in much smaller density differences between the surface and the nitracline (shallowest depth at which nitrate was detectable by conventional methods). The cumulative result of these two factors was that, compared to the surrounding Slope Water, less than one fifth as much wind energy would have been needed near the center of the ring to mix the water column from the nitracline to the surface. The altered nutrient and density profiles within the ring were consistent with physical and biological models of eddy dynamics, which predict that the frictional decay of the rotational flow field should result in shoaling of the main thermocline, and thus upwelling, in the interior of a warm-core eddy. There is reasonably good quantitative agreement between calculated rates of vertical nutrient transport associated with upwelling in the interior of the ring and the measured net uptake rates of nitrate and silicic acid by the phytoplankton.